Improve escaping
Improve escaping

- Add possibility to escape the escape character
- Move escaping to a separate function

Allows escaping the escape char if you need a backslash in front of the
substituted sequence. An even number of backslashes in front of the
sequence will lead to a substitution, is the number of backslashes odd,
the sequence will be escaped from parsing.

jbnicolai/envplate,kreuzwerker/envplate,kreuzwerker/envplate